To make the dough, combine the flour, sugar, and salt in a medium bowl. Add the butter and work into flour with fingertips until mixture starts to look like a coarse meal. Add enough cold water to bind the dough together.
Form dough into a ball and flatten out. Wrap in plastic wrap and refrigerate for at least 1 hour.
Preheat your oven to 400 F.
In a medium bowl, toss the plum slices with 4 tablespoons of the sugar, lemon juice, vanilla, and cinnamon. Set aside.
Roll out the dough to your desired thickness and size. Sprinkle with 1 tablespoon of the sugar. In the center of dough, lightly sprinkle 1 tablespoon flour.
Mound the plum mixture in the center of the dough, and reserve any remaining juices. Sprinkle the plums with the remaining tablespoon of sugar. Fold border of dough up around the plums like a free-form tart.
Bake in the oven until golden brown for about 30 minutes.
Meanwhile, pour the reserved juices into a small saucepan and bring to a slow boil. Reduce heat and simmer for about 3-4 minutes, or until mixture has thickened and slightly reduced. Brush plums and outer edges of crostada with mixture, if desired.